In the working section of the hydraulic machine arrangement there are, as a rule, parts moving relative to each other. These parts limit at least a pressure chamber the volume of which varies during one working cycle. The pressure in said working section is at least during a part of the working cycle higher than the pressure in the hollow. It is almost impossible to have the contact area between the moving parts absolutely tight so that a certain leakage occurs. Such leakage should be kept at a minimum. However, a certain leakage is acceptable, since this leakage forms a kind of lubrication flow.
The leakage is collected in the hollow and has to be transported away. To this end, the hollow is connected to a low pressure port of the supply pressure arrangement. When the hydraulic machine arrangement is in form of a pump, the hollow is often connected to a suction port of the supply port arrangement. In this way, it is avoided that a pressure builds up within the hollow.
The moving parts in the working section show some wear.